Transistorized quenching arrangement for a duration-controlled flash tube

ABSTRACT

A normally disabled transistor gate is serially connected with the main electrodes of a flash tube for selectively coupling operating voltage thereto. A reference voltage generated when an igniting pulse is applied to the tube is coupled to one input of a comparator circuit whose output excites a Schmitt trigger, thereby causing the trigger to generate a voltage of a first polarity. The trigger output voltage opens the gate to energize the main electrodes of the flash tube, thereby causing the tube to emit a flash of light. A portion of the light reflected from an object illuminated by the flash tube is detected by a photosensitive element and integrated to form a control signal, which is coupled to a second input of the comparator circuit. After a lapse of time sufficient to permit the control voltage to exceed the reference voltage, the comparator output reverses the state of the output voltage from the Schmitt trigger to disable the gate and terminate the flash.

UnitedStates Patent Dennewitz CONTROLLED FLASH TUBE Primary Examiner-Alfred C. Brody Attorney-Arthur 0. Klein Inventor: Rolf Dlctor Dennewitz, Berlin, Germany Assignee: Loewe Opt: GmbH, Berlin, Germany [57] ABSTRACT Fil d; m 6, 1970 A normally disabled transistor gate is serially connected with the main electrodes of a flash tube for selectively coupling APPL 611742 operating voltage thereto. A reference voltage generated when an igniting pulse is applied to the tube is coupled to one F i n m priority input of a comparator circuit whose output excites a Schmitt trigger, thereby causing the trigger to generate a voltage of a A 6 1969 G y 19 40 616-7 first polarity. The trigger output voltage opens the gate to energize the main electrodes of the flash tube, thereby causing :15. Cl ..315/l59, 315/241 P, 323/21 the tube to emit a flash oflight A portion of the light reflected 41/38 from an object illuminated by the flash tube is detected by a o are l I] 241 1 photosensitive element and integrated to form a control signal, which is coupled to a second input of the comparator circuit. After a lapse of time sufficient to permit the control References Cited voltage to exceed the reference voltage, the comparator out- UNITED STATES p ATENTS put reverses the state of the output voltage from the Schmitt trigger to disable the gate and terminate the flash. 3,350,604 10/1967 Erickson ..3l5/241- X 1/1971 Sceley .323/21 X 4 Claims, 1 Drawing Figure l I. l L r-'""" m 9 9a r l aN/r/n 7 fufgg l b 1 mm; 3

OEJ-QJ/ TRANSISTORIZED QUENCHING ARRANGEMENT FOR A DURATION- [451 July 4, 1972 TRANSISTORIZED QUENCHING ARRANGEMENT FOR A DURATION-CONTROLLED FLASH TUBE BACKGROUND OF THE INVENTION Conventional devices which automatically adjust the operating duration of a flash tube to obtain a predetermined quantity of emitted light employ a controllable switch (generally a quench tube of the gas discharge type) connected in shunt with the flash tube. The switch is triggered into conduction when the integrated light derived from the emission of the flash tube has reached a predetermined value. When the quench tube conducts, the exciting energy from the flash capacitor bypasses the flash tube and is dissipated in the quench tube.

The portion of the available energy for the flash tube that is dissipated in the quench tube represents wasted energy, i.e., energy that does not result in useful light output. Also, the successive charging and recharging of the flash capacitor to and from a completely or nearly completely discharged state following each flash and quench operation limits the maximum repetitive speed of the device and increases its recycling time.

I One manner of overcoming this disadvantage has been proposed in applicants co-pending application Ser. No. 874,935, filed Nov. 7, 1969, now US. Pat. No. 3,612,947 granted Oct. 12, 1971, and entitled Electronic Timing Apparatus for Controlling the Duration of Light Emission of a Flash Unit." In this arrangement a thyristor gate, of the interruptible type that can be switched both on and off by pulses of respectively opposite polarity applied to a control electrode thereof, is serially connected in the main energizing path of the flash tube. The thyristor gate is opened to switch the flash tube on by means of a Schmitt trigger or similar bipolar threshold device, which is excited by a reference voltage derived from an ignition pulse applied to a trigger electrode of the flash tube. A photosensitive element operatively associated with the light output of the flash tube generates a control signal that varies in proportion to the amount of light integrated. The output of the integrating circuit is also coupled to the input of the Schmitt trigger in such a manner that when the control signal has reached a predetermined value sufficient to overcome the previously established reference voltage at the input of the Schmitt trigger, the output polarity of the latter reverses to reclose the thyristor gate and terminate the flash.

SUMMARY OF THE INVENTION An alternative technique of this general type for overcoming the flash inefficiency and recycling time problems inherent in the use of shunting quench tubes is provided by the instant invention. In an illustrative embodiment, a normally disabled transistor gate is connected in series with the main electrodes of the flash tube for selectively coupling operating voltage thereto from a flash capacitor when the gate is opened. A Schmitt trigger which supplies bipolar output pulses to the base of the transistor to control its operating state has its input coupled to the output of a comparator circuit. A reference voltage derived from the ignition circuit of the flash tube when the ignition pulse is generated is applied to a first input of the comparator circuit while the control signal generated by the integrating circuit in response to reflected light from the illuminated object is applied to the second input.

With this arrangement, the comparator circuit opens the gate via the Schmitt trigger as soon as the ignition pulse is generated and keeps it open until the control voltage exceeds the reference voltage. At that point, the comparator circuit reverses the state of the Schmitt trigger to disable the transistor gate and terminate the flash.

This flash termination technique is effective to instantaneously stop the discharge of the flash capacitor, so that the latter does not have to be completely recharged from zero after each flash termination; thus, the recycling time of the unit is improved. Moreover, since only the exact amount of flash capacitor energy needed to produce useful light output from the tube is consumed, the arrangement is highly efficient.

BRIEF DESCRIPTION OF THE DRAWING The nature of the invention and its advantages will appear more fully from the following detailed description taken in conjunction with the appended drawing, whose single FIGURE depicts a transistorized quenching arrangement for a duration-controlled flash tube in accordance with the invention.

DETAILED DESCRIPTION Referring now to the drawing, there is illustrated a conventional flash tube 1 (illustratively a gas-filled envelope) which is provided with a pair of main excitation terminals 2 and 3. The

main terminals are coupled through the collector-emitter path of a normally disabled NPN transistor gate 4 and a resistor 5 to the terminals of a conventional flash capacitor 6. The junction of the resistor 5 and the flash capacitor 6 is illustratively at ground potential. The flash capacitor may be selectively charged in a known manner with a relatively high voltage of the polarity from a conventional DC. power source 7.

The tube 1 is further provided with a trigger electrode 8 which is coupled to an ignition circuit 9. The ignition circuit 9, whose operating potential may be provided by the flash capacitor 6 via a connection 9A, transmits an ignition pulse via an output 9B to the trigger electrode 8 upon closure of a pair of synchronizing contacts 10-10. The ignition circuit 9 also has an output 9C which exhibits a selectable reference voltage when the trigger pulse is generated. Further details of this type of ignition circuit may be found in applicants abovementioned copending application.

The voltage of the charged flash capacitor 6, which is coupled to the tube 1 when the transistor gate 4 is opened as described below, is not normally sufiicient by itself to cause emission of light energy from the tube 1. However, in a conventional manner such tube will emit light upon the concurrent application of an igniting pulse to the trigger electrode 8 by the ignition circuit 9. The resulting flash of light energy produced in the tube 1 is emitted through the walls of the tube envelope toward a suitable object 11. A portion of such light reflected from the object 1 1 is detected by a photosensitive resistor 12 and integrated by a grounded capacitor 13 in the manner described below. The photosensitive resistor 12 and the capacitor 13 define an integrating circuit whose input excitation voltage is taken across the resistor 5, so that the integrating circuit is conditioned for operation only when the flash tube 1 is in its conductive state.

As is well known, the maximum time duration of the flash of a conventional flash tube is essentially determined by the discharge characteristics of the associated flash capacitor through the flash tube. As will be described presently, the instant invention automatically adjusts the flash duration in a more rapid and efficient manner than heretofore in those instances where the object to be illuminated requires less than the maximum amount of light energy obtainable from the tube in a single flash.

The conductive state of the transistor gate 4 is controlled by the voltage at its base electrode. In the particular arrangement shown in the drawing, the gate 4 will be closed to maintain the flash tube 1 inoperative during the time that the base electrode (designated 13A) of the transistor is negative, and will open to couple the energizing voltage from the flash capacitor 6 to the tube 1 to condition the latter for operation when the base electrode 13A is made positive.

The base electrode 13A is coupled to the output of a bipolar control circuit 14 whose output polarity is normally negative to maintain the gate 4 closed. The control circuit 14 includes a conventional voltage comparator circuit 16 having a first input terminal 17 and a second input terminal 18. The output of the comparator circuit exhibits a first state (e.g., a prescribed output polarity) only when the voltage at the terminal 17 exceeds that at the terminal 18, and a second state (e.g., the opposite output polarity) when the voltage at the terminal 18 equals or exceeds that at the terminal 17, so that the comparator circuit is normally in its second state.

The output of the comparator circuit 16 is coupled to the base electrode 13A of the gate 4 through a Schmitt trigger 19. The output of the Schmitt trigger is assumed to be normally negative when the comparator circuit is in its second state, thereby maintaining the gate 4 normally closed as required.

The input terminals 17 and 18 of the control circuit 14 are respectively coupled to (a) the output 9C of the ignition circuit 9 via a resistor 21 and (b) the output of the photosensitive resistor 12. A second grounded capacitor 22 is connected at the junction of the terminal 17 and the resistor 21.

In use, the synchronizing contacts 10 of the flash device are initially open, and the input terminals 17 and 18 of the control circuit 14 are quiescent. The Schmitt trigger 19 therefore maintains a steady negative potential on the base electrode 13A of the transistor gate 4. The gate 4 is thus closed to prevent operating voltage from the flash capacitor 6 from exciting the main electrodes 2 and 3 of the flash tube 1.

When the synchronizing contacts 10 are momentarily closed, the reference voltage generated by the ignition circuit 9 on the output 9C is coupled to the input terminal 17 of the control circuit 14 via the resistor 21. Since the control voltage has not yet had time to build up across the capacitor 13 in the integrating circuit, the reference voltage at terminal 17 will be initially greater than the control voltage at terminal 18, whereby the comparator circuit 16 assumes its first state. This action reverses the negative output polarity of the Schmitt trigger 19. The resulting positive voltage applied to the base electrode 13A of the gate 4 opens the latter and couples operating voltage from the flash capacitor 6 to the main electrodes 2 and 3 of the flash tube 1.

The closure of the contacts 10 also causes the application of an igniting pulse to the trigger electrode 8 of the flash tube 1 via the output 98 of the ignition circuit 14. The combination of this igniting pulse and the now-present operating voltage across the main electrodes 2 and 3 drives the tube 1 into its conductive state and causes it to emit light energy. The conduction of the tube 1 causes current flow through the resistor 5, which generates excitation voltage for the integrating circuit including the photosensitive resistor 12 and the capacitor 13.

During the interval that the flash tube 1 emits light, the resistance of the photosensitive resistor 12 is lowered sufficiently, in a well-known manner, by the influence of the light reflected from the object 11, to charge the capacitor 13 from the voltage source represented by the potential across the resistor 5. As a result, the control voltage builds up across the capacitor 13. When the flash duration has persisted for a time long enough so that the control voltage at the input terminal 18 is equal to the reference voltage at the input terminal 17, the comparator circuit 16 again assumes its second state. The Schmitt trigger 19 will therefore again be reversed to apply a negative disabling voltage to the base electrode 13A of the gate 4. The resulting closure of the gate 4 stops the discharge of the flash capacitor 6 through the tube 1, and the latter ceases to emit light energy. The resulting cutoff of the tube 1 also disables the energizing voltage generated across the resistor for exciting the photosensitive resistor 12.

It will be noted that unlike flash duration controls of the prior art, the instant arrangement employs only so much of the energy stored on the flash capacitor 6 that is necessary to produce useful emitted light from the tube 1. In most instances, this will represent an incomplete discharge of the capacitor 6. Thus, the time necessary to restore full voltage on the capacitor 6 from the DC. power source 7 in the interval between successive flashes can be relatively short compared to the time necessary to fully recharge the capacitor 6 from a completely discharged state. Hence, rapid and efficient operation of the flash unit is assured with the instant arrangement.

Although the invention is described with reference to one preferred embodiment thereof, it is to be expressly understood that it is in no way limited to the disclosure of such a preferred embodiment, but is capable of numerous modifications within the scope of the appended claims.

What is claimed is: 1. In combination with a pulsed flash tube having a transconductive path and a trigger electrode and operable to emit a flash of light upon the application thereto of operating voltage through the transconductive path and the simultaneous application of an igniting pulse to the trigger electrode, an apparatus for automatically adjusting the operating duration of the flash tube to obtain a controllable quantity of light energy therefrom when the tube is ignited, which comprises:

a normally closed transistor gate having its collector-emitter path serially connected with the transconductive path of the flash tube and operative when open to apply operating voltage to the flash tube, the transistor gate having a base electrode responsive to signals of one polarity for opening the gate and responsive to signals of the opposite polarity for closing the gate;

first integrating means responsive to a portion of the light energy contained in the emission from the flash tube for generating a control signal that varies in proportion to the amount of light integrated;

means for selectively applying an igniting pulse to the trigger electrode;

second means rendered effective by the applying means for generating a reference signal;

bipolar control means coupled to the first and second generating means for initially producing an output signal of the one polarity when the tube is ignited and for subsequently terminating the last-mentioned signal and producing an output signal of the opposite polarity when the control signal has reached a predetermined value; and

means for coupling the output signal of the bipolar control means to the base electrode of the transistor gate.

2. Apparatus as defined in claim 1, in which the bipolar control means comprises, in combination, a comparator circuit having first and second inputs and an output exhibiting a voltage of one polarity when the voltage at the first input exceeds that at the second input and a voltage of the opposite polarity when the voltage at the second input exceeds that at the first input;

means for applying the reference signal to the first input;and

means for applying the control signal to the second input.

3. Apparatus as defined in claim 2, in which the bipolar control means further comprises a Schmitt trigger circuit coupled to the output of the comparator circuit and in which the coupling means comprises means for connecting the output of the Schmitt trigger to the base electrode of the transistor gate.

4. Apparatus as defined in claim 1 in which the first generating means comprises, in combination, a photosensitive resistor in operative association with the light emitted from the flash tube;

a capacitor;

means for coupling the photosensitive resistor to the capacitor, the control signal being available at the output of the capacitor; and

means connected in the transconductive path of the flash tube for electrically exciting the photosensitive resistor when the transistor gate is opened.

UNITED STATES PATENT OFFICE I CERTIFICATE OF CORRECTION Patent No. 3,675,074 Dated July 4, 1972 Inventor(s) Rolf Dieter Dennewitz v It is certifiedthat error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the cover sheet [72] "Rolf Dictor Dennewitz" should read Rolf Dieter Dennewitz Signed and sealed this 12th day of December 1 97 2.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting' Officer Commissioner of Patents F ORM PO-105O (10-69) USCOMM-DC 6Q376-P69 fl' UIS. GOVERNMENT PRINTING OFFICE: 1989 (P-365434,

UNITED STATES PATENT OFFICE CERTWICATE 0F CORRECTIGN Patent No. 3,675,074 Dated July 4, 1972 Inventor(s) Rolf Dieter Dennewitz It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the cover sheet [72] "Rolf Dictor Dennewitz" should read Rolf Dieter Dennewitz Signed and sealed this 12th day of December 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM PC4050 (10-69) USCOMM-DC 60376-P69 u.5. GOVERNMENT PRINTING OFFICE: was 0-366-334. 

1. In combination with a pulsed flash tube having a transconductive path and a trigger electrode and operable to emit a flash of light upon the application thereto of operating voltage through the transconductive path and the simultaneous application of an igniting pulse to the trigger electrode, an apparatus for automatically adjusting the operating duration of the flash tube to obtain a controllable quantity of light energy therefrom when the tube is ignited, which comprises: a normally closed transistor gate having its collector-emitter path serially connected with the transconductive path of the flash tube and operative when open to apply operating voltage to the flash tube, the transistor gate having a base electrode responsive to signals of one polarity for opening the gate and responsive to signals of the opposite polarity for closing the gate; first integrating means responsive to a portion of the light energy contained in the emission from the flash tube for generating a control signal that varies in proportion to the amount of light integrated; means for selectively applying an igniting pulse to the trigger electrode; second means rendered effective by the applying means for generating a reference signal; bipolar control means coupled to the first and second generating means for initially producing an output signal of the one polarity when the tube is ignited and for subsequently terminating the last-mentioned signal and producing an output signal of the opposite polarity when the control signal has reached a predetermined value; and means for coupling the output signal of the bipOlar control means to the base electrode of the transistor gate.
 2. Apparatus as defined in claim 1, in which the bipolar control means comprises, in combination, a comparator circuit having first and second inputs and an output exhibiting a voltage of one polarity when the voltage at the first input exceeds that at the second input and a voltage of the opposite polarity when the voltage at the second input exceeds that at the first input; means for applying the reference signal to the first input;and means for applying the control signal to the second input.
 3. Apparatus as defined in claim 2, in which the bipolar control means further comprises a Schmitt trigger circuit coupled to the output of the comparator circuit and in which the coupling means comprises means for connecting the output of the Schmitt trigger to the base electrode of the transistor gate.
 4. Apparatus as defined in claim 1 in which the first generating means comprises, in combination, a photosensitive resistor in operative association with the light emitted from the flash tube; a capacitor; means for coupling the photosensitive resistor to the capacitor, the control signal being available at the output of the capacitor; and means connected in the transconductive path of the flash tube for electrically exciting the photosensitive resistor when the transistor gate is opened. 